A variety of gas-barrier materials have heretofore been proposed. In recent years, however, from the environmental standpoint and as a gas-barrier material that is not affected by the humidity conditions, there has been proposed a gas-barrier material obtained by ionically crosslinking a polycarboxylic acid type polymer with a polyvalent metal.
For example, the following patent document 1 provides a gas-barrier resin composition which is characterized by reacting a polycarboxylic acid type polymer with a crosslinking agent having 2 to 4 functional groups reacting with the carboxy groups and a divalent or more metal ion to form crosslinked part due to the crosslinking agent and crosslinked part due to the divalent or more metal ion on the polycarboxylic acid type polymer, and having a polycarboxylic acid type polymer/crosslinking agent weight ratio of 99.9/0.1 to 65/35. Further, the following patent document 2 is proposing a gas-barrier film obtained by forming a gas-barrier coating on at least one surface of a thermoplastic resin film, the coating being formed of a polyacrylic acid crosslinked with a crosslinking agent that contains an epoxy compound having three or more epoxy groups per a molecule thereof, and the crosslinking agent being contained in an amount of 1 to 100 parts by mass per 100 parts by mass of the polyacrylic acid.
The gas-barrier materials disclosed in the above patent documents 1 and 2 must be crosslinked to a high degree by being heated at a temperature of as high as 150° C. or more or for an extended period of time, seriously affecting the plastic base body. Further, the gas-barrier materials require a dip treatment or a spray treatment for being ionically crosslinked with metal ions arousing, therefore, such problems as poor productivity while consuming large amounts of energy and water. Besides, the above gas-barrier materials are not fully satisfactory, either, in regard to flexibility and retort resistance.
As a gas-barrier laminated film that can be dry-baked at a relatively low temperature, further, there has been proposed a gas-barrier laminated film comprising a layer (a) formed by using a coating solution (A) containing a polycarboxylic acid type polymer without heat treatment, and a layer (b) formed by using a coating solution (B) containing a water-soluble polyvalent metal salt and an aqueous resin, the layer (a) formed of the coating solution (A) and the layer (b) formed of the coating solution (B) forming at least a pair of lamination units neighboring each other (patent document 3).